1. Field of the Invention
The present invention relates to a solar cell, and particularly to a multi-junction silicon thin-film solar cell.
2. Description of the Related Art
Solar cells are classified a silicon solar cell, a compound semiconductor solar cell, and an organic solar cell according to materials, and wherein they can be further subdivided into a bulk type, a thin-film type, a single crystal type and a poly-crystal type based on crystallization morphologies and elemental structures.
A general solar cell comprises a p-n junction structure formed by combining a p-type semiconductor and an n-type semiconductor. A strong electric field to pull electrons towards an n-type side and holes towards a p-type side occurs at the p-n junction. That is, at the p-n junction, electrons like children sliding down a slide and holes like bubbles in water floating up move towards opposite directions. In other words, the p-n junction functions to separate electrons and holes.
Although a compound semiconductor (fox example, CIGS or CdTe) solar cell has a higher conversion efficiency, manufacture of the compound semiconductor is costly due to use of rare element, and Cd and Te are toxic substances, easily resulting in environmental pollution. Consequently, the silicon solar cell remains being a mainstream of commercialized solar cells.
Today, the bulk-type crystal silicon solar cell has a great market share of solar cells. However, since the thin-film silicon solar cell has a thickness in semiconductor layer, which is 1/10˜ 1/100 of that of the bulk crystal silicon solar cell, it is prevented from the problem of the shortage of silicon raw material to which the production of the bulk crystal silicon solar cell is subjected, and it can be expected to greatly reduce costs. Therefore, it draws much attentions to be a next generation solar cell.
Further, in the thin-film silicon solar cells, the amorphous silicon (a-Si) solar cell is most popular. Although the a-Si solar cell has a high optical absorption coefficient and can be manufactured in several hundreds of nanometers in film thickness, its conversion efficiency is degraded by 10% under light irradiation. As compared with the a-Si solar cells, the crystal silicon (c-Si) thin-film solar cells (here, microcrystalline Silicon (μc-Si) or polysilicon (poly-Si) are totally referred to as crystal silicon), which can be manufactured in the same way as that of the a-Si solar cell, can absorb a light having a longer wavelength band without photodegradation. Therefore, a thin-film solar cell having a higher conversion efficiency can be manufactured by laminating the above-stated two solar cells. In FIG. 1, a conventional tandem cell with an a-Si solar cell functioning as a top cell and a c-Si solar cell functioning as a bottom cell, is shown. Since the top solar cell and the bottom solar cell can absorb light with short and medium wavelength bands of an incident light, respectively, in high efficiency, solar spectrum can be efficiently used with wider wavelength band being covered.
FIG. 1 shows a schematic structure of a conventional a-Si/c-Si tandem solar cell 1 having a p-i-n junction. The tandem solar cell 1 is formed with an a-Si cell 22 made of a p+-type a-Si layer 16, an n-type or i-type a-Si layer 18, and an n+-type a-Si layer 20; a transparent conductive oxide layer 24; a c-Si cell 32 made of a p+-type poly-Si or μc-Si layer 26, an i-type poly-Si or μc-Si layer 28, and an n+-type poly-Si or μc-Si layer 30; a metal electrode 34 made of Ag or Al; and a passivation layer 36 made of SiNx, on a glass substrate 12 having an anti-reflective coating 10 and a textured transparent conductive oxide layer 14, in order.
FIG. 2 is a flow chart of manufacturing the a-Si/c-Si tandem solar cell having a p-i-n junction as shown in FIG. 1. An anti-reflective coating (ARC) 10 is grown on a glass substrate 12 by high density plasma chemical vapor deposition (HDPCVD) or plasma enhanced chemical vapor deposition (PECVD) (step S10) to reduce the reflection of light on the solar cell surface. A transparent conductive oxide layer (TCO) 14, such as ITO or SnO2/ZnO, is grown by sputtering (step S20). In order to efficiently use a light incident to the cell, the transparent conductive oxide layer 14 is textured by wet etching (step S30). Next, a p+-type a-Si layer 16, an n-type or i-type a-Si layer 18, and an n+-type a-Si layer 20 in an amorphous silicon (a-Si) cell 22 are grown in order by HDPCVD (steps S40, S50, and S60). A transparent conductive oxide layer (TCO) 24, such as ITO or SnO2/ZnO, is grown by sputtering (step S70). Then, a p+-type poly-Si or μc-Si layer 26, an i-type poly-Si or μc-Si layer 28, and an p+-type poly-Si or μc-Si layer 30 in a crystal silicon (c-Si) cell 32 are grown in order by HDPCVD or PECVD (steps S80, S90, and S100). Next, a metal electrode 34 is formed by sputtering Ag or Al using an E-gun evaporator (step S110). Since the silicon surface of the solar cell is in a state that unstably generated carriers are easy to recombine (i.e., the surface recombination rate is large), SiNx is grown by HDPCVD or PECVD into a passivation layer 36 (step S120), thereby stabilizing the surface to suppress the losses caused by the surface recombination. Finally, the films are thermally processed by metal rapid thermal annealing (metal RTA) or rapid thermal annealing (RTA) (step S140), to enhance the densification and adhesion of the films.
However, very large contact resistances (referring to the resistances generated by the n+/TCO junction and the TCO/p+ junction), series resistances (referring to the inherent resistances of n+, TCO, and p+), carrier recombination, and inherent light transmittance of TCO exist in the above tandem solar cell having p+-i-(amorphous silicon)-n+/TCO/p+-i-(crystal silicon)-n+ structure, to cause a problem of low conversion efficiency.